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Reprinted  from   ECONOMIC  GEOLOGY,   Vol.  II,   No.   7,  October-November,   1907. 


THE   ASSOCIATION    OF   ALUNITE   WITH    GOLD    IN 
THE    GOLDFIELD    DISTRICT.  NEVADA  - 


FREDERICK   LESLIE   RANSOME 


Economic  Geology  Publishing;  Company 


^U^fO 


THE  ASSOCIATION   OF  ALUNITE   WITH   GOLD   IN 
THE   GOLDFIELD    DISTRICT,    NEVADA.^ 

Frederick  Leslie  Ransome. 

INTRODUCTION. 

Alunite,  a  hydrous  sulphate  of  potassium  and  aluminium  with 
the  formula  K2O.3AI2O3.4SO3.6H2O,  is  one  of  those  minerals 
the  apparent  rarity  of  which  is  probably  due  in  no  small  measure 
to  inconspicuousness.  The  material  was  first  noted  at  Tolfa,  near 
Rome,  where  it  was  used  as  early  as  the  fifteenth  century  for 
the  manufacture  of  potassium  alum  (K20.Al203.4S03.24H20).^ 
This  salt,  containing  2  parts  less  of  AI2O3  with  18  parts  more 
of  water,  is  obtained  from  the  alunite  by  roasting  and  lixiviation. 
Alunite  is  found  also  on  some  of  islands  of  the  Grecian  Archi- 
pelago, near  Muszay  in  Hungary,  on  Mount  Dore  in  France  and 
in  a  few  other  localities  in  Europe,  as  well  as  with  the  opal  of 
Queretaro,  in  Mexico.  In  the  United  States  it  has  been  described 
in  the  Rosita  Hills,^  on  Calico  Peak  near  Rico,^  in  the  National 
Belle  Mine  near  Silverton,^  and  at  Cripple  Creek,^  all  in  Colo- 

'  Published  with  the  permission  of  the  Director  of  the  U.  S.  Geological 
Survey. 

*  For  the  latest  account  of  this  celebrated  locality,  with  a  bibliography,  see 
L.  De  Launay,  "  La  Metallogenie  de  ITtalie."  Tenth  Geological  Congress, 
Mexico,  1906,  pp.  125-132. 

'  Cross,  Whitman,  "  Geology  of  Silver  Cliff  and  the  Rosita  Hills,  Colorado." 
17th  Ann.  Rept.  U.  S.  Geol.  Survey,  Pt.  2,  1896,  pp.  52-56.  Also  Am.  Jour. 
Set.,  3d  ser.,  vol.  41,  1891,  pp.  466-475. 

*  Cross  and  Spencer.  "  The  Geology  of  the  Rico  Mountains,  Colorado." 
2ist  Ann.  Rept.  U.  S.  Geol.  Survey,  Pt.  2,  1900,  pp.  92-94. 

'Hurlbut,  E.  B.  "On  Alunite  from  Red  Mountain,  Ouray  County,  Colo." 
Am.  Jour.  Set.,  3d  ser.,  vol.  48,  1894,  pp.  130-131.  Also  Ransome,  F.  L.  "A 
Report  on  the  economic  geology  of  the  Silverton  Quadrangle,  Colorado." 
Bull.  U.  S.  Geol.  Survey  No.  182,  1901,  p.  235. 

*  Lindgren  and  Ransome.  "  Geology  and  Gold  Deposits  of  the  Cripple 
Creek  District,  Colorado."  Profess.  Paper  U.  S.  Geol.  Survey  No.  54,  1906, 
p.  125. 

667 


668  FREDERICK   LESLIE  RANSOME 

rado.  It  occurs  also  at  Tres  Cerritos,  Mariposa  County,  Cali- 
fornia.^ 

Mr.  W.  Lindgren^  found  alunite  in  the  Ryerson  mine  near 
Morenci,  Arizona,  associated  with  pyrite  and  kaolin.  He  re- 
garded it  (page  193)  as  formed  by  the  attack  of  descending  sul- 
phuric acid  solutions  upon  sericite  away  from  free  oxygen. 

Until  recently  geologists  have  regarded  alunite  as  formed 
exclusively  by  the  attack  of  sulphurous  fumarolic  vapors  upon 
feldspatchic  rocks.^  At  Cripple  Creek,  however,  the  mineral  was 
found  only  in  the  oxidized  ore,  and,  like  the  kaolinite  of  that 
district,  is  probably  secondary  with  reference  to  the  original  tellu- 
ride  ores.  De  Launay,*  moreover,  has  recently  maintained  with 
cogency  that  the  alunite  of  Tolfa  is  probably  not,  as  formerly 
supposed,  a  solfataric  product,  but  is  due  to  the  action  of  perco- 
lating surface  water,  charged  with  sulphuric  acid  by  the  oxida- 
tion of  pyrite,  upon  a  particularly  feldspathic  facies  of  the  trach- 
yte. It  is  accordingly  no  longer  justifiable  to  regard  the  occur- 
rence of  alunite  as  proof  of  former  fumarolic  activity.  The  min- 
eral may  form  under  very  different  sets  of  conditions  and  it  is 
necessary,  in  seeking  the  explanation  of  any  occurrence  of  it,  to 
keep  alternative  hypotheses  in  mind. 

In  all  four  of  the  Colorado  localities  the  alunite  occurs  in 
regions  which  contain  ore  deposits.  In  the  Rosita  Hills  and  in 
the  Rico  and  Silverton  districts  it  is  apparently  not  a  product 
of  superficial  oxidation.  No  close  kinship  between  this  mineral 
and  the  ores,  however,  has  been  shown,  although  it  was  doubtless 
recognized  by  all  the  workers  in  those  districts  that  there  might 
be  some  genetic  relation  between  the  metallic  sulphides  and  the 
volcanism  of  which  the  sporadic  solfataric  metamorphism  of 
feldspathic  rocks  to  quartz-alunite  aggregates  was  one  manifes- 
tation.    It  is  worth  noting  in  this  connection  that  the  indexes  of 

*  Turner,  H.  W.     "  Rocks  and  Minerals  from  California."     Am.  Jour.  Sci., 
4th  ser.,  vol.  5,  1898,  pp.  424-426. 

•"The  Copper  Deposits  of  the  CI  if  ton- Morenci  District,  Arizona."     Profess. 
Paper  U.  S.  Geol.  Survey  No.  43,  1905,  pp.  1 19-120,  169  and  193-194. 

*  Iddings,  J.  P.     "Rock  Minerals."     New  York,  1906,  p.  476. 

*  Of>.  cit. 


ASSOCIATION   OF  ALUNITE    WITH   GOLD  669 

two  of  the  best  and  most  recent  works^  on  ore  deposits  contain 
no  reference  to  alunite,  showing  that  the  mineral  has  not  been 
commonly  recognized  as  one  intimately  associated  with  ores. 
Neither  is  it  mentioned,  so  far  as  I  am  aware,  in  President  C.  R. 
Van  Hise's  work  on  metamorphism.^  Professor  J.  H.  L.  Vogt,^ 
however,  in  discussing  metasomatic  vein-processes,  has  remarked : 
"  I  would  mention  also  the  formation  of  alum-stone ;  quartz- 
alunite  rocks ;  quartz-diaspore  rocks,  etc. ;  and  also  the  formation 
of  bauxites,  etc.  But  I  do  not  know  that  these  changes  have 
been  anywhere  observed  in  genetic  relation  with  ore-veins." 

In  the  Goldfield  district  alunite  is  so  abundant  and  so  gener- 
ally associated  with  the  intense  alteration  accompanying  ore 
deposition  that  it  must  be  looked  upon  as  one  of  the  most  charac- 
teristic minerals  of  the  gold  deposits.  This  close  association  of 
the  gold  with  a  mineral  which  apparently  had  not  been  recog- 
nized in  Nevada  prior  to  the  present  investigation  and  which  has 
been  reported  in  comparatively  few  localities  elsewhere,  seems 
worthy  of  some  description,  especially  as  the  relation  promises 
to  throw  light  upon  the  origin  of  the  ores. 

ROCKS   OF  THE  DISTRICT. 

For  a  preliminary  account  of  the  geology  of  Goldfield  the 
reader  is  referred  to  an  earlier  publication.^  A  full  description 
of  the  geology  and  ore  deposits  is  in  preparation  and  will  appear 
as  one  of  the  professional  papers  of  the  U.  S.  Geological  Survey. 
It  is  sufficient  in  this  place  to  state  that  the  ores  occur  in  middle 
or  early  Tertiary  rhyolite,  latite,  andesite  and  dacite.  Nearly  all 
of  these  are  extrusive  flows,  but  the  dacite,  which  contains  most 
of  the  important  mines,  is  an  intrusive  sheet,  although  the  intru- 
sion probably  took  place  beneath  a  cover  of  volcanic  rocks  which 

*  Beck,  Richard.  "  Lehre  von  den  Erzlagerstatten."  Berlin,  1901.  Stelzner- 
Bergeat.      '  Die  Erzlagerstatten."     Leipzig,  1904-1906. 

* "  A  Treatise  on  Metamorphism."     Monog.  U.  S.  Geol.  Survey  No.  47,  1904. 

* "  Problems  in  the  Geology  of  Ore  Deposits."  Trans  Amer.  Inst.  Mining 
Engineers,  vol.  31,  1902,  p.  150. 

•Ransome,  F.  L.  "Preliminary  Account  of  Goldfield,  Bullfrog,  and  Other 
Mining  Districts  in  Southern  Nevada."  Bull.  U.  S.  Geol.  Survey  No.  303, 
1907. 


670  FREDERICK  LESLIE  RANSOME 

was  not  of  great  thickness.  The  ore-bearing  rocks  rest  upon, 
or  cut  through,  a  pre-Tertiary  foundation  of  granitic  rock  with 
which  is  involved  some  Cambrian  shale,  and  are  themselves,  in 
outlying  parts  of  the  district,  unconformably  overlain  by  tuffa- 
ceous  lake  sediments  and  later  Tertiary  lavas. 

Although  comparatively  unaltered  representatives  of  most  of 
the  rocks  named  may  be  found  within  the  district,  yet  the  meta- 
morphism  which  they  have  undergone  over  much  of  the  area  is 
of  a  most  conspicuous  kind.  Dark  pyroxene-andesites,  dacite, 
and  rhyolite  have  alike  been  changed  to  nearly  white  aggregates 
of  secondary  minerals.  Such  products,  as  a  rule,  retain  only 
traces  of  the  original  textures  of  the  igneous  rocks  from  which 
they  have  been  derived.  To  detect  and  interpret  these  traces 
requires  patient  field  observation  of  the  progressive  stages  of 
alteration,  supplemented  by  microscopical  study. 

TYPES   OF  ALTERATION. 

Three  types  or  stages  of  alteration  are  recognizable.  Where 
the  chemical  activity  has  been  most  intense  the  rocks  have  been 
changed  to  porous,  fine-grained  aggregates  consisting  essentially 
of  quartz.  This  is  the  material  of  which  are  composed  the  hun- 
dreds of  cragg>^  points  and  ledges  which  are  one  of  the  most 
characteristic  features  of  the  topography  of  the  district.  Although 
most  of  these  ledges,  so  far  as  known,  contain  no  ore  bodies,  yet, 
until  recently,  no  ore  has  been  found  that  was  not  in  or  along- 
side such  quartz  rock.  Some  of  the  discoveries  made  in  1906, 
however,  show  that  very  rich  bodies  of  ore  may  occur  at  places 
where  there  is  no  siliceous  outcrop. 

The  second  type  of  alteration  is  one  in  which  the  rock  is 
changed  to  a  comparatively  soft,  light-colored  mass  of  quartz, 
kaolinite,  alunite  and  pyrite.  At  any  given  locality  the  boun- 
dary between  the  rocks  representing  the  two  kinds  of  alteration 
may  be  fairly  sharp;  but  the  proportions  of  quartz,  kaolinite  and 
alunite  vary  widely,  and  consequently,  over  the  field  at  large, 
gradations  in  composition  and  hardness  may  be  found  between 
these  two  typical  metamorphic  products.     Both  types  are  usually 


ASSOCIATION  OF  ALUNITE    WITH   GOLD  671 

associated  with  the  ores  and  are  most  conspicuously  displayed 
over  a  belt  of  country  stretching  east  from  Goldfield  past  Preble 
Mountain.  This  mountain,  in  fact,  is  composed  chiefly  of  ande- 
site  and  dacite  which  have  undergone  alteration  of  the  kinds 
descrijDcd. 

The  third  type  of  alteration  has  effected  less  conspicuous 
results  than  the  other  two  and  its  products  are  not  closely  asso- 
ciated with  the  ores.  It  consists  in  the  development  of  calcite, 
quartz,  chlorite,  epidote  and  pyrite  at  the  expense  of  the  original 
minerals  and  groundmass  and  is  thus  propylitic  in  character.  It 
is  rather  sharply  marked  off  from  the  second  type  by  the  presence 
of  calcite  and  the  absence  of  alunite,  these  minerals  not  having 
yet  been  observed  together  in  the  same  specimen.^  Rocks  which 
have  undergone  this  form  of  alteration,  chiefly  andesite,  latite 
and  dacite,  still  retain  something  of  their  original  color  and 
texture,  the  principal  external  sign  of  change  being  a  greenish 
tint  instead  of  the  usual  dark  gray  of  the  fresh  rock,  and  a  lack 
of  luster  in  the  phenocrysts. 

GENERAL  OCCURRENCE  OF  THE  ALUNITE. 

Attention  was  first  directed  to  the  occurrence  of  alunite  in  the 
district  by  the  finding  of  small  nests,  or  crystalline  aggregates, 
up  to  five  millimeters  in  diameter,  of  a  pale  pink  mineral  in  a 
greatly  altered  nearly  white  rock  exposed  on  the  east  shoulder 
of  Vindicator  Mountain,  about  two  miles  northeast  of  the  town 
of  Goldfield.  The  rock  contains  quartz  phenocrysts  and  was 
probably  originally  a  rhyolite.     The  microscope  shows  that  the 

*  Since  this  was  written,  very  finely  crystalline  calcite,  sericite  and  probably 
some  kaolinite  have  been  found  associated  with  much  larger  crystals  of  alunite 
in  an  altered  rhyolite  from  the  east  slope  of  Vindicator  Mountain.  The 
molecular  ratio  of  soda  to  potash  in  the  alunite,  as  determined  by  Dr.  Hille- 
brand,  is  as  40  to  45.  It  is  possible,  however,  that  the  result  obtained  for 
potash  may  be  high  in  consequence  of  some  decomposition  of  the  sericite  by 
the  sulphuric  acid  used  to  dissolve  the  alunite. 

The  occurrence  together  of  alunite  and  calcite,  apparently  developed  simul- 
taneously, shows  that  the  alteration  took  place  under  conditions  which  did  not 
permit  the  free  escape  of  carbon  dioxide.  It  is  scarcely  conceivable,  for  ex- 
ample, that  calcite  could  form  in  the  presence  of  sulphuric  acid  percolating 
down  from  an  overlying  deposit  of  oxidizing  pyrite. 


EXPLANATION   OF   PLATE  IX. 

Fig.  I.  Photomicrograph  of  thin  section  of  altered  dacite  (G.  398)  from 
D  level  of  the  Combination  Mine,  Goldfield.  Magnified  40  diameters.  Nicols 
crossed. 

The  section  shows  a  characteristic  pseudomorphous  aggregate  of  alunite 
iA),  and  quartz  (Q)  after  a  labradorite  phcnocryst.  In  this  case  there  is 
an  isotropic  material  (O)  present  which  is  probably  opal.  The  groundmass 
is  a  fine-grained  aggregate  of  quartz,  alunite  and  pyrite,  the  crj-stals  of  the 
last  mineral  (black)  being  in  some  places  grouped  in  pseudomorphous  aggre- 
gates after  hornblende  and  biotite. 

Fig.  2.  Photomicrograph  of  thin  section  of  altered  dacite  (G.  634)  from 
the  200- foot  level  of  the  Florence  Mine,  Goldfield  Magnified  40  diameters. 
Ordinary  light. 

The  section  shows  rather  more  alteration  than  the  preceding,  little  of  the 
original  texture  of  the  dacite  remaining.  Pyrite  (black)  is  abundant  and  is 
crystallized  with  alunite  {A).  The  latter  mineral  exhibits  characteristic 
lath-shaped  sections  with  irregular  ends  and  felty  aggregation.  It  has  evi- 
dently developed  in  some  places  along  devious  cracks.  There  is  much  quartz 
(Q)  in  the  rock  but  it  is  for  the  most  part  too  finely  crystalline  to  appear 
clearly  in  the  photograph. 


672 


Plate  IX. 


Economic  Geology. 


FiQ   1. 


Fig.  2. 


ASSOCIATION   OF  ALUNITE    WITH   GOLD  673 

feldspar  phen.ocrysts,  as  well  as  the  groundmass,  have  been 
altered  to  an  aggregate  of  quartz  and  alunite,  the  optical  deter- 
mination of  the  latter  mineral  being  checked  by  independent 
chemical  tests  in  the  Survey  laboratory  by  Dr.  W.  F.  Hillebrand 
and  Dr.  W.  T.  Schaller.  With  the  alunite  is  associated  a  subor- 
dinate quantity  of  an  anhedral/  colorless  mineral  of  high  refrac- 
tive index,  strong  birefringence  and  possessing  a  single  well- 
marked  cleavage.  The  mineral  is  biaxial  and  optically  negative, 
the  plane  of  the  optic  axes  coinciding  with  the  cleavage.  It 
resembles  diaspore  but  differs  from  that  mineral  in  being  optically 
negative.  It  has  not  yet  been  identified.  Undoubted  diaspore 
has  been  found  intergrown  with  alunite,  in  the  altered  dacite  of 
the  Florence  mine. 

Microscopic  study  of  the  bleached,  altered  rocks  of  the  district 
has  revealed  alunite  in  nearly  all  of  them.  The  rhyolite  forming 
the  summit  of  Columbia  Mountain,  just  north  of  the  towns  of 
Goldfield  and  Columbia,  has  been  changed  to  a  quartz-alunite 
rock,  the  alunite,  with  some  quartz,  forming  pseudomorphs  after 
the  feldspar  phenocrysts  which  probably  ^vere  originally  ortho- 
clase.  Alunite  is  very  abundant  in  the  altered  andesite  and  dacite 
of  Preble  Mountain  and  vicinity  and  is  present  in  practically  all 
of  the  altered  dacite  which  forms  the  country  rock  of  the  Com- 
bination, Florence,  January,  Red  Top,  Jumbo  and  other  mines 
near  Goldfield.  It  also  occurs  in  abundance  directly  associated 
with  the  sulphide  ores  in  these  mines,  in  some  places,  particularly 
in  the  January  mine,  as  a  soft,  easily  pulverized  material,  of  snow 
white  or  faint  pink  color,  which  is  not  always  distinguishable  on 
mere  inspection  from  certain  pure  forms  of  kaolinite  which  occur 
in  the  mines  of  this  and  other  districts.  At  Goldfield,  the  deli- 
cate pink  tint  of  the  alunite  is  rather  characteristic  and  serves 
often  as  a  rough-and-ready  means  of  distinguishing  it  from  the 
kaolinite  with  which  it  is  often  associated.  Close  inspection  of 
the  altered  rocks  in  which  alunite  is  abundant  usually  reveals 

*AnhedraI  =  without  crystal  faces,  xenomorphic,  allotriomorphic.  See 
Cross,  Iddings,  Pirsson  and  Washington,  "The  Texture  of  Igneous  Rocks," 
Journal  of  Geology,  vol.  14,  1906,  p.  698. 


674  FREDERICK  LESLIE  RANSOME 

this  faint  pink  color  in  the  aggregates  which  have  replaced  the 
original  feldspar. 

As  a  constituent  of  altered  rocks  in  the  Goldfield  district,  the 
alunite  does  not  have  complete  crystal  form.  It  is  tabular  in 
habit,  the  basal  pinacoid  being  well  developed  and  the  rhombo- 
hedral  faces  being  absent.  In  thin  sections  it  shows  distinct 
cleavage  parallel  to  the  basal  planes  and  parallel  extinction  with 
reference  to  the  cleavage.  The  birefringence  is  apparently 
rather  stronger  than  the  difference  y  —  a  =  0.018  given  in  the 
table  of  Levy  and  Lacroix.^  the  interference  colors  in  good  thin 
sections  being  mostly  yellow  but  rising  in  places  to  red  or  blue 
of  the  first  order.  The  mineral  gives  a  positive  uni-axial  inter- 
ference figure,  in  sections  showing  no  cleavage.  The  index  of 
refraction  is  distinctly  higher  than  the  balsam  of  the  slide  (1.54), 
that  given  for  the  alunite  of  Tolfa  being  ••=1.572  and  e=  1.592.* 
The  most  decisive  optical  means  of  distinguishing  the  mineral 
from  the  optically  negative  sericite  which  it  resembles  in  habit  and 
refraction,  is  by  means  of  the  quartz  wedge  or  gypsum  plate. 
Sericite  shows  the  higher  interference  colors  when  the  section 
is  turned  with  the  cleavage  at  right  angles  to  the  direction  of 
greatest  elasticity  (X)  of  the  plate  or  wedge,  while  alunite  shows 
the  brighter  colors  when  the  basal  cleavage  is  parallel  to  this 
direction. 

The  soft  massive  alunite,  such  as  occurs  in  ilie  January  mine, 
when  gently  crushed  on  a  microscopical  slide  and  stirred  in  a 
drop  of  water,  shows  under  a  high  power  of  the  microscope  thin, 
colorless  scales  of  hexagonal  outline.  The  average  diameter  of 
those  examined  is  less  than  0.05  millimeter,  and  such  is  their 
tenuity  that  no  crystal  faces  can  be  recognized  on  the  edges  of 
the  basal-pinacoidal  scales. 

COMPARISON   OF   FRESH   AND   ALTERED   DACITE. 

Inasmuch  as  the  largest  and  richest  ore  bodies  are  in  the  dacite 
and  as  the  rock  is  uniform  in  character,  it  was  chosen  for  a  close 


1  (I 


Les  Mineraux  des  Roches."     Paris,  1888.     Also  Iddings,  J.  P.     "  RocV 
Minerals."     New  York  and  London,  1906. 
"Iddings,  J.  P.     Op.  cit.,  p.  476. 


ASSOCIATION  OF  ALUNITE    WITH   GOLD  6ys 

study  of  the  alteration  effected  by  the  ore-depositing  agencies. 
Such  comparison  is  especially  favored  by  the  fact  that  the  most 
intense  alteration  is  often  local  and  consequently  it  is  possible  to 
obtain  specimens  of  fresh  and  altered  rock  within  moderate  dis- 
tances of  each  other.  The  two  specimens  of  dacite  analyzed  and 
compared  wxre  taken  about  one  and  one  fourth  miles  apart. 

The  unaltered  and  typical  dacite  is  a  dark  gray  rock  with 
abundant  phenocrysts  of  plagioclase,  up  to  a  centimeter  in  length, 
lying  in  a  dark  aphanitic  groundmass.  There  are  present,  also, 
less  numerous  phenocrysts  of  augite  and  biotite  with  occasional 
anhedral  grains  of  quartz.  The  volumetric  ratio  of  phenocrysts 
to  groundmass  is  estimated  at  rather  less  than  i  to  3. 

Under  the  microscope  the  feldspar  phenocrysts  prove  to  be 
labradorite  of  the  approximate  composition  AbiAnj.  They  are 
perfectly  fresh,  with  slightly  rounded  euhedral^  outlines.  A 
noticeable  feature  is  the  presence  in  many  crystals  of  closely 
crowded  minute  inclusions  of  glass  which  in  some  cases  consti- 
tute cloudy  cores  or  kernels,  in  others  sharply  bounded  shells,  the 
center  of  the  crystal  being  clear.  The  augite  crystals  are  roughly 
euhedral  or  subhedral,  with  a  maximum  measured  extinction 
angle,  Z  t\C,  of  about  45°.  The  biotite  occurs  in  the  usual 
short,  rather  irregular  pseudo-hexagonal  prisms.  Both  biotite 
and  augite  are  entirely  fresh.  The  quartz  phenocrysts  are  anhe- 
dral, with  rounded,  embayed  or  sharply  angular  outlines. 

Hornblende  is  very  abundant  but  is  confined  almost  exclu- 
sively to  the  groundmass,  most  of  the  crystals  being  comparable 
in  size  rather  with  the  feldspar  microlites  than  with  the  pheno- 
crysts just  described.  The  largest  hornblende  crystal  in  the  thin 
section  studied  is  about  half  a  millimeter  in  length.  It  should 
be  said,  however,  that  in  dacite  from  other  parts  of  the  district 
the  hornblende  occurs  as  phenocrysts. 

The  feldspar  microlites  in  the  groundmass  are  at  least  in  part 
labradorite  of  as  calcic  composition  as  the  phenocrysts.  They, 
the  hornblende,  and  a  little  magnetite  and  apatite,  are  imbedded 
in  an  abundant  glass,  which  in  transmitted  light  is  gray,  with 

*  Euhedral  =  bounded  by  crystal  faces ;  idiomorphic,  automorphic. 


676  FREDERICK  LESLIE  RANSOME 

incipient  crystals,  or  crystallites.     One  crystal  of  titanite,  0.3 
millimeter  in  length,  is  in  the  thin  section  studied. 

A  chemical  analysis  of  this  rock,  made  by  Mr.  George  Steiger 
in  the  Survey  laboratory,  is  given  under  I.  in  the  table  on  page  680. 

The  arbitrary  norm  of  this  rock,  calculated  from  analysis  in 
accordance  with  the  plan  of  the  "  quantitative  classification,"^  is 
as  follows: 

Norm  of  DAaxE. 

Quartz 17.34 

Orthoclase  15.01 

Albite 25.68 

Anorthite  21.96 

Zircon  37 

Diopside 5.21 

Enstatitc  4-63 

Magnetite  4-87 

Ilmenite    1.52 

Apatite   ...  .34 

(Water 2.95) 

99.88 

As  the  rock  contains  much  undifferentiated  glass  no  close  cal- 
culation of  its  real  mineralogical  composition,  or  mode,  is  pos- 
sible. The  norm  and  chemical  analysis  show  that  the  rock  is  a 
tonalose  in  the  terminology  of  the  quantitative  classification. 

The  altered  dacite,  of  which  a  chemical  analysis  by  Mr.  George 
Steiger  is  given  under  II.,  was  taken  from  the  D  level  of  the 
Combination  Mine,  about  235  feet  below  the  surface  and  in  a 
part  of  the  mine  where  no  apparent  oxidation  had  taken  place. 
The  original  water  level  in  this  mine  appears  to  have  been  at 
about  130  feet  from  the  surface,  near  which  depth  the  oxidized 
ore  gives  place  to  sulphides.  The  rock  analyzed  is  the  typical 
wall-rock  of  this  and  adjacent  mines. 

*  Cross,  Iddings,  Pirsson  and  Washington.  "  The  Quantitative  Classification 
of  Igneous  Rocks."     Chicago  and  London,  1903. 

For  the  benefit  of  the  reader  who  may  not  have  had  occasion  to  familiarize 
himself  with  modern  petrology  it  may  be  explained  that  the  "  norm  '*  is  a 
classificatory  expedient  and  is  usually  very  different  from  the  actual  mineral 
composition,  or  "mode."  The  dacite  for  instance  really  contains  no  ortho- 
clase, the  potassium  being  combined  in  biotite  and  in  the  glass  of  the  ground- 


ASSOCIATION   OF  ALUNITE    WITH   GOLD  677 

The  altered  rock  is  light-gray,  flecked  with  numerous  dull 
white  spots  which  represent  the  original  feldspar  phenocrysts. 
The  few  quartz  phenocrysts  are  unchanged,  but  the  ferromag- 
nesian,  or,  more  briefly,  femic,  minerals  have  wholly  disappeared. 
Pyrite,  in  small,  disseminated  crystals,  is  abundant. 

Under  the  microscope,  it  is  seen  that  the  only  original  con- 
stituents are  the  quartz  phenocrysts,  even  these  showing  some 
development  of  pyrite  along  irregular  microscopic  cracks,  and  an 
occasional  small  crystal  of  zircon.  The  feldspars  are  changed 
to  aggregates  of  minutely  crystalline  kaolinite  and  tabular  crys- 
tals of  alunite,  the  latter  being  sometimes  arranged  in  sheaf-like 
bunches.  (See  Plate  IX.)  The  alunite  has  the  form  character- 
istic of  its  occurrence  in  this  district,  sections  cut  at  large  angles 
with  the  basal  pinacoid  having  the  shape  of  narrow  laths  with 
irregular  ends.  The  length  of  these  lath-shaped  sections  rarely 
exceeds  half  a  millimeter.  The  alunite,  when  cursorily  exam- 
ined under  a  low-power  lens,  may  easily  be  mistaken  for  sericite 
(muscovite),  although  of  course  this  error  is  quickly  dispelled 
on  closer  study,  sericite  being  optically  negative  and  alunite 
positive.  The  interference  colors,  moreover,  although  bright, 
particularly  in  the  larger  crystals,  are  of  a  lower  order  than  those 
of  sericite.  Quartz  also  accompanies  the  alunite  and  kaolinite  in 
some  of  the  pseudomorphs  after  labradorite.  (See  Plate  IX., 
Fig.  I.) 

The  former  femic  constituents  can  be  recognized  from  the  fact 
that  they  contain  little  or  no  kaolinite,  the  quartz  and  alunite 
being  here  associated  with  abundant  pyrite.  In  some  cases  the 
pseudomorphous  aggregate  still  preserves  a  suggestion  of  the 
former  cleavage  of  the  biotite  or  of  the  outlines  of  hornblende. 

The  groundmass  is  changed  to  a  fine-grained  aggregate  of 
kaolinite,  quartz,  alunite  and  pyrite.  The  last-named  mineral 
occurs  also  in  irregular  microscopic  veinlets. 

As  the  rock  is  wholly  crystalline  and  is  made  up  of  minerals  of 
approximately  constant  composition,  it  is  possible  to  calculate 
with  some  precision  the  actual  mineralogical  constitution  from 
the  chemical  analysis.     This,  in  percentages  of  mass,  is  as  follows  : 


678  FREDERICK  LESLIE  RANSOME 

MiNERALOCICAL  G)MPOSITION  OP  ALTERED  DaCITE. 

Quartz  ( SiOj)   49-38  per  cent,  of  mass. 

Kaolinite  (AU0a.2Si0r2H,0)  23.99 

Alunite'   (IG0.3Al,Os4S0..6H,0) 157.^ 

Pyrite   (FeSi)    7.^ 

Water  (H,0)    ..  25.^ 

Other  constituent;* ...     1.17 

loaoo 


Compared  as  they  stand,  the  two  analyses  show  close  agree- 
ment in  silica,  alumina,  titanic  oxide  and  phosphorous  pentoxide. 
When  all  the  iron  in  both  analyses  is  calculated  as  ferrous  oxide, 
that  of  the  altered  rock  shows  a  loss  of  only  0.7  per  cent.,  so  that 
in  this  respect  also  the  two  analyses  are  very  close  together.  On 
the  other  hand,  the  analysis  of  the  altered  rock  shows  a  loss  of 
practically  all  the  lime  and  magnesia,  most  of  the  soda  and  one 
half  of  the  potash.  It  has  gained  a  large  quantity  of  combined 
water,  nearly  6  per  cent,  of  sulphuric  anhydride  and  nearly  4  per 
cent,  of  sulphur. 

Such  a  direct  comparison  of  analyses,  however,  while  indi- 
cating in  this  case  the  general  character  of  the  change  that  has 
taken  place  in  the  dacite,  is  really  a  comparison  of  equal  masses 
of  the  two  rocks,  or,  more  strictly  speaking,  of  the  two  powders 
as  prepared  and  weighed  for  analysis,  and  as  Mr.  Lindgren* 
pointed  out  some  years  ago,  would  accurately  portray  the  char- 
acter of  the  alteration  only  under  two  conditions,  namely,  (i) 
that  the  specific  gravity  or  density  of  the  rock  is  unchanged  by 
the  metamorphism,  and  (2)  that  the  alteration  has  not  produced 
any  change  of  volume  in  the  rock  mass  as  a  whole,  such  a  change, 
for  example,  as  takes  place  when  peridotite  alters  to  serpentine. 
It  is  essential  in  an  investigation  of  this  kind  to  determine  what 
materials  have  been  added  to  or  subtracted  from  a  unit  volume 

*  The  molecular  proportions  of  potash  and  soda  as  calculated  from  analysis 
II.,  page  680,  are  respectively  11  and  14.  All  of  the  potash  and  8  of  the  soda 
molecules  are  needed  to  satisfy  the  75  molecules  of  S0».  Consequently  the 
Goldfield  alunite  is  a  soda-bearing  variety. 

' "  Metasomatic  Processes  in  Fissure-veins."  Trans.  Am.  Inst.  Mining 
Eng„  vol.  30,  1901,  pp.  591-595. 


ASSOCIATION   OF  ALUNITE    WITH  GOLD  6yg 

(or  unit  mass)  of  fresh  rock  in  order  to  produce  a  given  altera- 
tion product. 

The  changes  undergone  by  a  given  volume  of  rock  can  be 
determined  most  simply  if  the  alteration  is  unaccompanied  by 
swelling  or  contraction  of  the  whole  rock  mass.  The  altered 
dacite  shows  no  indication  of  increase  of  bulk.  The  field  evi- 
dence of  such  swelling  would  comprise  irregular  fracturing  and 
squeezing  of  the  rock,  the  presence  of  curved  slickensided  surfaces, 
and  pressure  effects  in  the  microscopical  texture  of  the  altered 
dacite.  Contraction  would  probably  be  more  difficult  to  detect. 
Any  notable  degree  of  shrinkage,  however,  would  be  likely  to 
record  itself  by  the  development  of  many  irregular  open  cracks 
or  by  some  modification  of  the  original  texture  of  the  rock,  such 
as  the  distortion  of  phenocrysts.  Evidence  of  either  kind  being 
absent,  it  may  be  concluded  with  some  confidence  that  whatever 
volumetric  change  has  taken  place  is  to  be  sought  in  a  compari- 
son of  the  porosity  of  the  two  rocks  and  not  in  change  of  total 
bulk. 

The  porosity  of  the  fresh  dacite  is  exceedingly  slight  and  is 
not  apparent  under  the  microscope.  An  accurate  specific  gravity 
determination  of  the  rock  powder  used  for  analysis,  made  by  Mr. 
George  Steiger,  gave  the  figures  2.654.^  He  also  determined  the 
specific  gravity  of  the  whole  hand  specimen  as  2.63,  the  differ- 
ence between  the  two  results  being  an  approximate  measure  of 
the  porosity.  The  specific  gravity  of  the  altered  dacite,  which 
is  noticeably  porous,  as  calculated  from  the  mineralogical  com- 
position on  page  6yy,  assigning  to  the  1.17  per  cent,  not  accounted 
for  the  average  specific  gravity  of  the  other  constituents  is 
2.769.^  Determinations  by  Mr.  Steiger  gave  2.766  for  the  pow- 
dered rock  and  2.49  for  the  hand  specimen.     These  figures  indi- 

*  The  specific  gravities  of  the  powders  were  determined  by  placing  weighed 
quantities  in  water  in  a  tared  picnometer  and  boiling  them  at  low  temperature 
under  reduced  pressure.  They  were  then  kept  for  some  hours  in  a  thermostat 
and  quickly  weighed  at  25**  C.  The  specific  gravities  of  the  rock  masses  were 
obtained  by  weighing  them  in  air  and  quickly  in  water.  The  figure  2.49,  ob- 
tained for  the  altered  dacite,  is  probably  slightly  too  high  in  consequence  of 
some  absorption  by  pores  open  to  the  surface. 

'The  figures  used  for  the  specific  gravities  of  the  minerals  were:  Quartz 
2.66,  kaolinite  2.61,  alunite  2.60,  and  pyrite  5.0. 


68o 


FREDERICK  LESLIE  RANSOME 


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ASSOCIATION   OF  ALUNITE    WITH   GOLD  68 1 

cate  that  the  substance  of  the  altered  rock  has  gained  4.2  per  cent, 
in  weight  and  that,  notwithstanding  this  gain,  a  given  volume 
of  rock,  under  the  supposition  of  constant  volume,  has  suffered 
a  net  loss  of  5.3  per  cent,  in  weight  consequent  upon  an  increase 
of  porosity. 

Since  the  volume  of  a  given  mass  of  rock,  in  cubic  centimeters, 
is,  for  all  practical  purposes,  equal  to  its  weight  in  grams  divided 
by  its  specific  gravity,  the  percentage  of  pore  space  in  the  fresh 
dacite  is  obtained  from  the  equation 

287.4  _  287.4  ^^874  ^ 
2.63        2.654         2.63      ' 

where  287.4  is  the  weight  of  a  specimen  of  dacite  in  air,  2.63 
the  specific  gravity  of  the  rock  mass,  2.654  the  specific  gravity 
of  the  poreless  mineral  constituents,  and  x  the  proportion  of  pore 
space  which  in  this  case  is  equal  to  0.9  per  cent.  In  the  altered 
rock,  the  proportion  of  pore  space  is  9.9  per  cent.,  showing  an 
increase  in  porosity  of  9  per  cent,  of  the  initial  volume. 

The  chemical  changes  undergone  by  the  dacite  are  shown  in 
the  accompanying  table.  Columns  P.  and  IP.  are  not  essentia] 
as  regards  the  process  illustrated  although  they  represent  a  step 
in  the  reasoning.  They  are  of  value  also  as  showing  that  specific 
gravity  determinations  of  rock  powders,  while  useful  in  calcu- 
lations of  the  mineral  compositions  of  rocks,  are  of  minor  impor- 
tance in  a  comparison  of  this  kind,  in  which  it  is  necessary  to 
know  the  specific  gravity  of  the  mass  as  a  whole,  including  its 
pores.  Incidentally,  it  may  be  remarked  that  when  the  specific 
gravity  of  a  rock  is  given  in  connection  with  its  chemical  analysis, 
the  method  by  which  the  density  was  determined  should  be 
indicated. 

In  columns  P.  and  IP.  are  given  the  number  of  grams  of  each 
constituent  in  100  cubic  centimeters  of  each  rock,  the  figures 
being  obtained  by  multiplying  the  percentage  figures  in  columns 
I*,  and  IP.  by  the  respective  specific  gravities  of  the  fresh  and 
altered  dacite.  The  gains  and  losses  of  each  constituent  are 
given  in  grams  in  column  III.,  and  in  percentages  of  total  initial 


682  FREDERICK  LESLIE  RAN  SO  ME 

mass  in  column  IV.  The  last  column,  V.,  g^ves  the  percentage 
of  loss  of  each  constituent  and  perhaps  most  clearly  displays  the 
nature  of  the  change. 

It  appears  that  the  rock  has  lost  a  little  of  its  silica,  more  of 
its  alumina,  nearly  all  of  its  magnesia  and  lime,  three  fourths  of 
its  soda  and  two  thirds  of  its  potash.  On  the  other  hand,  the 
water  has  greatly  increased,  a  large  amount  of  sulphuric  anhy- 
dride has  been  added  and  the  iron  has  been  converted  to  pyrite. 
The  iron  oxides  in  column  P.  correspond  to  11.08  grams  of 
metallic  iron,  while  the  pyrite  in  column  IV.  corresponds  to  8.4 
grams  of  iron  and  9.6  grams  of  sulphur.  The  quantity  of  iron 
originally  in  the  dacite  was  therefore  not  only  sufficient  to  form 
all  the  pyrite  in  the  altered  rock  but  has  suffered  a  loss  of  over 
27  per  cent. 

Comparisons  of  fresh  and  altered  rocks  are  sometimes  made 
by  assuming  the  constancy  of  one  constituent  and  recalculating 
the  analysis  of  the  altered  rock  accordingly.  Such  a  method,  in 
the  present  case,  would  give  erroneous  results. 

The  metamorphic  agent  was  evidently  a  strongly  acid  solution 
carrying  sulphydric  and  sulphuric  acids,  and  possibly  sulphurous 
acid.  Whether  the  sulphuric  acid  which  was  doubtless  the  direct 
agent  in  the  formation  of  the  alunite  was  present  in  the  original 
solution  from  the  first,  or  whether  it  was  derived  from  the  hydro- 
gen sulphide,  by  oxidation  at  some  stage  in  the  movement  of 
the  solution  toward  its  point  of  attack  upon  the  rocks  now  visible, 
is  an  undecided  question.  The  absence  of  sulphates  from  contact 
ore-deposits  indicates,  however,  that  oxygenated  sulphur  com- 
pounds are  not  given  off  directly  from  solidifying  magmas.  The 
solution  was  capable  of  decomposing  the  silicates  in  the  rocks, 
carr>'ing  part  of  their  constituents  away,  reacting  with  the  iron 
of  the  magnetite  and  silicates  to  form  pyrite,  with  the  potas- 
sium and  aluminium  to  form  alunite  and  with  the  aluminium  to 
form  kaolinite.  The  large  percentage  of  loss  in  calcium,  potas- 
sium and  sodium  shows  that  the  active  solution  was  far  below 
its  saturation  point  for  these  elements  and  was,  therefore,  entirely 
different  in  character  from  the  supposedly  alkaline  solutions 
which  deposited  the  large  class  of  sericitic  and  calcitic  gold- 


ASSOCIATION   OF  ALUNITE    WITH   GOLD  683 

quartz  veins^  exemplified  by  those  of  California.  Before  pro- 
ceeding farther  with  genetic  considerations  I  shall  briefly  describe 
the  mineralogical  character  of  the  Goldfield  ores. 

THE   GOLD   ORES. 

The  typical  unoxidized  ore  as  it  occurs  in  the  dacite  near  Gold- 
field  consists  of  pyrite,  bismuthinite  and  a  reddish-gray  cuprifer- 
ous mineral  having  the  general  composition  of  tetrahedrite. 
Native  gold  is  usually  associated  with  these  minerals,  particu- 
larly with  the  bismuthinite  and  tetrahedrite,  and  in  the  rich  ore 
may  be  easily  visible.  These  minerals  are  often  arranged  in 
successive  layers  or  crusts  around  silicified  fragments  of  dacite, 
the  inner  crust  being  usually  the  richer. 

The  brownish  copper-bearing  mineral,  here  provisionally  called 
tetrahedrite,  contains  antimony  and  sulphur,  as  chemically  deter- 
mined by  Dr.  W.  T.  Schaller,  and  is  therefore  not  bornite, 
although  it  goes  by  that  name  in  the  district.  It  appears  so 
commonly  as  a  constituent  of  the  best  ore  that  there  is  a  strong 
suggestion  that  it  is  auriferous,  particularly  as  the  microscope 
fails  in  some  specimens  of  reputed  rich  ore  to  reveal  any  free 
gold.  Further  investigation  of  the  mineral  is  in  progress.  It 
may  possibly  prove  to  be  the  orthorhombic  species  famatinite 
which  contains  the  same  constituents  as  tetrahedrite,  but  in  dif- 
ferent proportions,  and  is  characterized  by  a  reddish  tint.  Bis- 
muthinite, the  sulphide  of  bismuth,  is  also  regarded  by  the  miners 
as  a  sign  of  rich  ore. 

Concentric  shells  of  ore  minerals  about  greatly  altered  rock- 
fragments  are  rather  characteristic  of  the  best  ore.  Specimens 
from  the  Combination  mine  show  an  inner  zone  of  free  gold  and 
quartz  up  to  an  eighth  of  an  inch  in  thickness.  This  is  covered 
by  a  shell  of  tetrahedrite  and  this  by  an  outer  crust  of  pyrite. 
In  the  Florence  mine  similar  fragments  show,  first,  a  shell  of 
pyrite,  then  one  of  tetrahedrite,  and,  finally,  a  thick  crust  of 
quartz  speckled  with  native  gold,  tetrahedrite  and  pyrite,  and 
transfixed  by  needles  of  bismuthinite.     Some  of  the  particles  of 

*See  Lindgren,  W.      "  Metasomatic  Processes  in  Fissure  Veins."      Trans. 
Am.  Inst.  Mining  Eng.,  vol.  30,  1901,  p.  668. 


684  FREDERICK  LESLIE  RANSOME 

gold  are  embedded  in  the  compact  quartz,  others  are  inclosed  in 
the  bismuthinite.  The  different  crusts  are  not  in  every  case 
sharply  defined  nor  are  they  necessarily  continuous. 

The  native  gold  of  these  ores  is  often  in  particles  so  fine  and 
so  closely  crowded  in  the  gangue  that  the  precious  metal  resem- 
bles a  streak  of  yellow  ocher.  The  proportion  of  silver  is  rather 
small,  the  average  of  thirty-four  assays  of  rich  ore  from  the 
Mohawk  mine,  made  for  Mr.  J.  W.  Finch,  giving  330  fine  ounces 
of  gold  and  46.5  ounces  of  silver  to  the  ton. 

Tests  and  chemical  analyses  of  the  ores  made  for  scientific  and 
technical  purposes  nearly  all  show  a  little  tellurium,  although  no 
tellurium  mineral  has  been  recognized  in  the  mines  close  to  Gold- 
field.  Tellurides  of  gold,  not  sufficiently  well  crystallized  for  a 
determination  of  their  species,  occur  in  the  Jumbo  Extension  and 
Goldfield-Belmont  mines,  near  Diamondfield,  about  four  miles 
northeast  of  Goldfield,  and  both  tellurite  and  emmonsite  (or  dur- 
denite)  are  found  in  the  oxidized  ore  of  neighboring  properties.' 

A  small  speck  of  sphalerite  was  found  by  Mr.  W.  H.  Black- 
burn in  the  Goldfield-Belmont,  but  galena  has  not  been  noted  in 
any  of  the  mines  so  far  as  known.  Chalcopyrite,  a  common  min- 
eral in  most  mining  districts,  was  not  found  at  Goldfield  in  the 
course  of  the  present  investigation,  but  it  is  said  to  occur  sparingly 
in  the  Sandstorm  and  Florence  mines. 

The  common  gangue  of  the  unoxidized  ore  is  quartz.  This  is 
usually  compact,  almost  flinty,  in  texture,  although  porous  in 
mass,  and  in  most  cases  bears  unmistakable  evidence  of  having 
resulted  from  the  silicification  of  dacite,  rhyolite,  or  andesite. 
Large  vugs  and  conspicuously  crystalline  quartz,  such  as  are 
found  in  typical  veins  the  world  over,  are  practically  absent  from 
the  Goldfield  district,  where  the  free  development  of  quartz  crys- 
tals in  open  spaces  is  represented  only  by  drusy  films,  lining  pores 
left  by  the  solution  of  phenocrysts  or  incrusting  small  interstitial 
cavities  in  brecciated  material. 

Associated  with  the  quartz  in  much  of  the  ore  are  soft  white 
substances,  such  as  the  miners  commonly  call  "  talc."     Chemical 

*  Ransome,  F.  L.  "  Preliminary  Account  of  Goldfield,  etc."  Bull.  U.  S. 
Geol.  Survey  No.  303,  1907,  p.  36. 


ASSOCIATION   OF  ALUNITE    WITH   GOLD  685 

and  microscopical  tests  of  the  white  material  show  that  in  most 
cases  it  is  either  alunite  or  kaolinite  or  a  mixture  of  the  two.^ 
Sericite,  supposed  when  the  preliminary  report^  on  the  district 
was  written  to  be  also  abundant,  proves  on  further  study  to  be 
very  rare  and  is  certainly  nowhere  an  abundant  or  characteristic 
mineral  in  the  ore  deposits.  Gypsum,  while  not  known  with 
actual  sulphide  ore,  is  very  abundant  in  the  altered  rocks  within 
a  hundred  feet  or  so  from  the  surface  and  does  occur  crystallized 
with  quartz  and  pyrite  on  the  Goldfield-Belmont  mine,  below  the 
belt  of  oxidation. 

As  a  rule,  the  gold  and  auriferous  sulphides  are  embedded  in 
quartz.  Alunite,  however,  is  often  so  close  to  the  gold  as  to 
appear  in  the  same  microscopic  field  and  the  telluride  of  gold  in 
the  Jumbo  Extension  mine  is  partly  embedded  in  alunite. 

The  ores,  it  should  be  noted,  do  not  occur  in  typical  veins  but 
in  masses  of  shattered,  altered  rock  of  very  irregular  shape.  In 
small  part  only  do  they  appear  to  have  filled  visibly  open  cavities 
or  fissures. 

For  descriptions  of  the  oxidized  ores,  which  have  been  of  so 
much  importance  at  Goldfield,  the  reader  is  referred  to  the 
advance  report  cited  and  to  the  forthcoming  memoir  on  the 
district.  In  some  places  they  contain  alum  derived  from  the 
alunite. 

THE   ORE-DEPOSITING   SOLUTIONS. 

The  mineralogical  character  of  the  Goldfield  ores  and  the 
alteration  of  the  wall  rock  show  that  the  solutions  or  vapors 
which  deposited  them  carried  gold,  copper,  bismuth,  antimony, 
a  little  arsenic  and  tellurium,  hydrogen  sulphide,  and  probably 
sulphurous  and  sulphuric  acids.  The  solvent  action  of  the  solu- 
tions upon  quartz  was  comparatively  slight,  as  shown  by  the  fact 
that  the  original  phenocrysts  of  the  dacite  still  retain  their  char- 
acteristic embayed  outlines  when  the  rest  of  the  rock  is  altered 

^  Mr.  E.  A.  Collins,  in  his  recent  good  description  of  the  Combination  mine 
(Mining  and  Scientiiic  Press,  vol.  95,  1907,  P-  398)  refers  to  the  ore  as  "a 
mixture  of  soft  kaolinized  material  and  hard  dacite."  The  "  kaolinized  ma- 
terial "  is  chiefly  alunite. 

•Bull.  U.  S.  Geol.  Survey  No.  303.  1907,  p.  35- 


686  FREDERICK  LESLIE  RANSOME 

to  quartz,  alunite  and  kaolinite.  The  quartz  set  free  by  the 
decomposition  of  the  silicates  has  for  the  most  part  recrystallized 
as  fine-grained  aggregates.  A  small  proportion,  as  we  have 
seen,  has  been  carried  away  in  solution,  perhaps  in  part  to  be 
recrystallized  in  small  neighboring  fissures,  and  in  part  permeat- 
ing the  surrounding  less  altered  rocks.  Large  masses  of  coarsely 
crystalline  vein  quartz  are  absent  from  this  district  and  it  seems 
necessary  to  conclude  that  the  ore-bearing  solutions  were  not 
only  poor  in  silica  but  that,  in  spite  of  their  energetic  attack  upon 
the  rocks,  they  were  not  good  solvents  for  quartz.  The  appar- 
ently intense  silicification  of  the  country  rock  in  proximity  to 
fissures  appears  at  first  glance  to  point  to  a  diflferent  conclusion. 
The  conversion  of  a  rock  containing  60  per  cent,  of  silica  to  a  fine- 
grained, pyritic,  quartz  aggregate  does  not,  however,  necessarily 
imply  the  direct  addition  of  40  per  cent,  of  silica.  The  result 
may  be  attained  by  the  removal  of  most  of  the  bases,  the  con- 
version of  the  iron  to  pyrite  and  the  development  of  a  porous 
texture  such  as  is  favorable  for  ore  deposition  and  is  character- 
istic of  the  Goldfield  ledges.  Moreover,  it  is  probable  that  some 
of  the  silica  shown  to  have  been  abstracted  from  the  altered  dacite 
may  have  been  concentrated  in  the  immediate  vicinity  of  the 
fissures.  Certainly,  whatever  silica  was  originally  in  the  solu- 
tions did  not  travel  far  from  the  fissures  through  which  they  rose. 

Whether  the  solutions  were  notably  ferruginous  is  doubtful. 
As  compared  with  other  districts,  the  pyrite  at  Goldfield  is  not 
abundant  and  all  of  that  in  the  country  rock  can,  as  has  been 
shown,  be  accounted  for  without  any  addition  of  iron.  There 
has,  in  fact,  been  an  abstraction  of  iron  which  may  have  been 
sufficient  to  form  the  pyrite  in  the  actual  ore,  upon  the  supposi- 
tion that  the  dissolved  iron,  like  the  silica,  migrated  toward  the 
fissures. 

In  general  the  heavy  metals  brought  in  by  the  solutions 
remained  within  or  very  close  to  the  multitude  of  small,  irregular 
fissures  that  afforded  opportunity  for  the  depositional  process. 
The  sulphur  acids,  on  the  other  hand,  penetrated  the  wall-rock 
for  considerable  distances. 


ASSOCIATION   OF  ALUNITE    WITH   GOLD  687 

It  is  very  probable  that  the  ore-bearing  solutions  contained 
carbon  dioxide,  although  the  ores  and  the  rock  adjacent  to  them 
are  free  from  carbonates.  Field  evidence  in  this  district,  as  well 
as  in  the  Cceur  d'Alene  district^  in  Idaho,  indicates  that  this  con- 
stituent, when  of  volcanic  origin,  becomes  active  in  the  formation 
of  carbonates  only  at  comparatively  low  temperature  and  that  it 
tends  to  penetrate  to  greater  distances  from  the  source  of  supply 
than  some  of  'the  other  constituents.  The  third,  or  propylitic 
type  of  alteration  described  on  page  671,  is  thought  to  be  largely 
due  to  the  extensive  permeation  of  the  rocks  by  solutions  con- 
taining carbon  dioxide.  It  thus  constitutes  an  outer  aureole  to  the 
more  intense  metasomatism  associated  directly  with  the  ores.  I 
hope  at  a  future  time  to  study  in  detail  the  relation  of  the  meta- 
morphism  particularly  described  in  this  paper  to  that  of  this  outer 
zone. 

It  may  be  recalled,  in  this  connection,  that  Sainte-Claire  Deville 
and  Leblanc,2  in  their  studies  of  Italian  volcanic  gases,  found 
that  sulphurous  and  carbonic  acids  never  occurred  together, 
although  the  association  of  sulphydric  and  carbonic  acids  was 
frequent.  They  concluded  that,  as  a  rule,  the  emanation  of 
carbon  dioxide  marks  a  later  and  cooler  stage  of  solfataric  activ- 
ity. The  transition  from  one  type  to  the  other,  however,  can 
hardly  be  abrupt. 

Just  what  became  of  the  lime  and  magnesia  removed  from  the 
altered  dacite  has  not  been  determined.  A  part,  at  least,  of  the 
calcium  was  probably  taken  into  solution  as  hydrous  sulphate 
and  deposited  in  fissures  as  gypsum,  this  mineral  being  known 
to  occur  in  the  district  in  places  where  its  formation  can  scarcely 
have  resulted  from  the  action  of  surface  waters.  Still  another 
part  and  some  of  the  magnesium  may  have  been  carried  as  car- 
bonates into  the  rocks  affected  by  metamorphism  of  the  third  or 
propylitic  type  and  there  deposited  metasomatically  as  calcite  or 
dolomite.     It  has  not  yet  been  ascertained,  however,  whether 

*  Ransome  and  Calkins.  "  The  Geology  and  Ore  Deposits  of  the  Coeur 
d'Alene  District."     Profess.  Paper  U.  S.  Geol.  Survey.     In  press. 

'"Memoire  sur  la  composition  chimique  des  gaz  rejetes  par  les  events 
volcaniques  de  I'ltalie  meridionale."  Ann.  de  Chimie  et  de  Phys.,  vol.  52, 
1858,  p.  45. 


688  FREDERICK  LESLIE  RANSOME 

there  has  actually  been  any  addition  of  calcium  or  magnesium  to 
these  carbonatized  rocks. 

Two  general  hypotheses  are  entertainable  with  reference  to 
the  source  of  these  solutions.  They  either  came  from  above,  the 
sulphurous  or  sulphuric  acid  having  been  derived  from  overlying 
deposits  of  oxidizing  sulphides,  or  they  came  from  below.  In 
the  former  case  the  solutions  were  presumably  cold ;  in  the  latter 
case  they  were  in  all  probability  hot.  The  difficulties  in  the  way 
of  the  descensional  hypothesis,  however,  appear  to  be  insur- 
mountable. 

All  available  geological  evidence  indicates  that  the  ore  deposits 
of  Goldfield  were  formed  comparatively  near  the  surface  and  that 
consequently  they  do  not  represent  the  final  stage  of  a  long  period 
of  erosion  and  progressive  downward  enrichment.  The  irregu- 
larity and  sporadic  occurrence  of  the  ore  bodies  would  seem  to 
be  alone  sufficient  to  render  any  such  long-continued  enrichment 
impossible.  We  are  driven  then  to  suppose  that  if  descending 
solutions  effected  the  intense  metasomatic  action  described,  these 
must  have  derived  their  chemical  energy  from  the  oxidation  of 
moderate  quantities  of  pyrite  comparatively  near  at  hand.  Under 
such  circumstances,  the  effect  produced  appears  to  be  greatly 
disproportioned  to  the  available  agency.  Such  a  process  might 
account  for  the  development  of  kaolin  and  perhaps  some  alunite 
within  the  oxidized  ore,  but  hardly  for  the  extensive  metasoma- 
tism of  the  country  rock  which  is  not  limited  to  the  vicinity  of 
known  ore  bodies  and  continues  far  below  all  traces  of  oxidation 
as  measured  by  the  presence  of  iron  oxides.  For  it  is  to  be  borne 
in  mind  that  the  alteration  described  in  this  paper  is  character- 
istically associated  with  unoxidized  ores  and  that  gold,  pyrite, 
tetrahedrite,  bismuthinite  and  the  other  minerals  found  in  such 
ores  were  deposited  at  the  same  time  that  the  neighboring  rock 
was  changed  to  quartz,  alunite  and  kaolinite.  If  the  alunitiza- 
tion  is  the  work  of  cold  descending  solutions  then  the  rich  ores 
of  Goldfield  are  entirely  the  product  of  the  same  agency.  In 
the  light  of  our  present  knowledge  of  ore  deposition,  this  seems 
improbable. 


ASSOCIATION   OF  ALUNITE    WITH   GOLD  689 

The  intensity  of  the  alteration  and  the  character  of  the  solu- 
tions, as  shown  by  the  composition  of  the  ores  and  by  the  meta- 
somatism of  the  country  rock,  point  to  hot  ascending  waters  as 
the  effective  agent.  Even  the  alunite,  in  spite  of  the  occurrence 
of  the  mineral  in  the  oxidized  ore  of  some  of  the  Cripple  Creek 
mines  and  the  view  held  by  De  Launay  with  regard  to  the  Tolfa 
occurrence,  is  suggestive  of  more  intense  chemical  activity  than 
is  usually  manifested  by  cold  oxidizing  solutions.  The  Gold- 
field  region,  moreover,  is  known  to  have  been  the  scene  of  vol- 
canic activity  throughout  most  of  Tertiary  time.  That  the  solu- 
tions were  essentially  emanations  from  a  solidifying  body  of 
dacitic  magma — the  reservoir  whence  came  the  intrusive  dacite 
and  the  extrusive  dacite  vitrophyre  of  the  district,  is  regarded  as 
a  probable  hypothesis.  It  is  significant  in  this  connection  that  a 
chemical  analysis  of  the  glassy,  effusive  dacite  vitrophyre  shows 
3.35  per  cent,  of  water  driven  off  below  110°  C.  and  4.06  per 
cent,  lost  above  that  temperature,  a  total  of  7.41  per  cent.  The 
intrusive  dacite  on  the  other  hand  contains  2.95  per  cent,  of 
water,  2  per  cent,  being  driven  off  above  110°  C.  The  dacitic 
magma  thus  originally  contained  a  large  quantity  of  water  and 
that  much  of  this  water  has  been  expelled  during  crystallization 
is  shown  by  the  difference  of  over  4  per  cent,  between  the  water 
of  the  two  analyses. 

The  limits  of  a  brief  paper  do  not  permit  of  a  full  discussion 
of  the  question  of  the  thickness  of  the  overlying  rocks  at  the 
time  the  ores  were  deposited.  General  geological  evidence  which 
will  be  set  forth  in  a  later  publication  indicates  that  the  ores  were 
deposited  at  comparatively  shallow  depth. 

ORIGIN   OF   KAOLINITE. 

Some  discussion  has  been  waged  in  the  literature  of  ore 
deposits  regarding  the  origin  of  kaolin  and  Mr.  Lindgren^  has 
recently  intimated  that,  although  he  himself  formerly  referred 
to  it  as  characteristic  of  certain  classes  of  veins,  kaolinite  should 

'"The  Relation  of  Ore-deposition  to  Physical  Conditions."  This  journal, 
vol.  II.,  1907,  p.  120. 


690  FREDERICK  LESLIE  RANSOME 

not  be  considered  as  a  gangue  mineral  of  any  class  of  ore  de- 
posits except  those  formed  under  the  influence  of  oxidation.  In 
a  subsequent  paragraph  he  somewhat  modifies  this  statement  by 
the  expression  of  belief  that  "  kaolin  is  rarely  formed  by  alkaline 
hot  water  at  any  considerable  depth  below  the  surface." 

Much  kaolin  originally  described  by  various  authors  as  formed 
by  the  solutions  which  first  brought  up  the  ores  associated  with 
it,  has  later  been  proved  to  have  resulted  from  oxidizing  proc- 
esses, it  being  one  of  the  most  characteristic  products  of  the 
action  of  percolating  acid  solutions  upon  aluminous  rocks.  The 
kaolin  at  Cripple  Creek  is  a  good  example.  Much  so-called  kaolin, 
moreover,  has  been  shown  to  be  sericite,  and  some  is  perhaps 
alunite.  These  facts,  however,  seem  scarcely  to  warrant  the  total 
elimination  of  kaolinite  from  the  list  of  gangue  minerals  found  in 
unoxidized  ore  deposits.  The  mineral  undoubtedly  occurs  as  an 
original  constituent  of  some  of  the  ores  of  the  San  Juan  Moun- 
tains, Colorado.*  Mr.  W.  H.  Weed^  has  described  it  as  a  product 
of  the  metasomatic  action  of  the  waters  of  Boulder  Hot  Springs, 
Montana,  upon  granite,  and  according  to  Mr.  S.  F.  Emmons' 
the  mineral  is  an  abundant  original  constituent  of  the  ore  of  the 
Bassick  mine.  Kaolinization  has  also  been  noted  by  Bela  von 
Inkey*  in  the  dacitic  country  rock  of  the  Nagyag  veins,  the  mate- 
rial being  afterwards  investigated  by  F.  Kollbeck,**  whose  analysis 
indicates  a  mixture  of  kaolinite  and  sericite. 

At  Goldfield,  the  intimate  association  of  the  kaolinite  with  the 
alunite,  gold  and  sulphides  shows  that  all  were  formed  at  the 
same  time  and  by  one  general  process  which  was  anterior  to  and 
entirely  independent  of  oxidation  or  weathering.     These  latter 

'  Ransome,  F.  L.  "  Economic  geology  of  the  Silverton  Quadrangle,  Colo." 
Bull.  U.  S.  Geol.  Survey  No.  182,  1901,  p.  y^. 

'"Mineral  Vein  Formation  at  Boulder  Hot  Springs,  Montana."  21st  Ann. 
Report  U.  S.  Geol.  Survey,  1900,  part  2,  p.  253. 

• "  Geology  of  Silver  Cliflf  and  the  Rosita  Hills,  Colorado."  By  Whitman 
Cross.  Accompanied  by  a  paper  on  the  mines  of  Custer  County,  Colorado. 
By  S.  F.  Emmons,  17th  Ann.  Rept.  U.  S.  Geol.  Survey.  1896,  part  2,  p.  432. 

*  "  Nagy4g  und  seine  Erzlagerstatten."     Budapest,  1885,  p.  143. 

• "  Untersuchungen  iiber  die  Zersetzung  des  Quarztrachyts  neben  Gold- 
erzgangen  von  Nagyag."  Oesterr.  Zeits.  f.  Berg-  und  Huttenwesen,  vol.  z'^* 
1888,  pp.  25-27. 


ASSOCIATION   OF  ALUNITE   WITH  GOLD  691 

processes  have  probably  also  resulted  in  the  formation  of  some 
kaolin,  but  with  this  secondary  development  the  present  paper  is 
not  concerned  beyond  the  pointing  out  of  the  distinction  between 
the  two  modes  of  genesis. 

CONCLUSION. 

The  recognition  of  alunite  as  a  characteristic  constituent  of  the 
Goldfield  ores  and  the  demonstration  of  its  genetic  relation  to 
them  establishes  a  new  type — that  of  alunitic  and  kaolinitic  gold- 
quartz  veins,  jn  the  classification  of  epigenetic  deposits  based 
upon  the  kind  of  metasomatism  effected  in  the  wall-rock  by  the 
ore-depositing  solutions.  While  the  Goldfield  deposits  are  prob- 
ably too  irregular  in  form  to  come  under  the  usual  definition  of 
vein,  yet  in  all  that  relates  to  genesis  of  the  ores  there  is  no  essen- 
tial difference  between  them  and  what  is  usually  termed  a  meta- 
somatic  fissure-vein. 

It  is  not  believed  that  the  Goldfield  district  is  unique  in  the 
possession  of  this  type.  Other  examples  are  likely  to  be  found 
among  the  great  number  of  ore  deposits  associated  with  Tertiary 
volcanism,  particularly  when  more  investigators  of  mining  dis- 
tricts realize  the  importance  of  close  studies  of  rock  alteration, 
such  as  have  been  so  admirably  carried  out  in  this  country  by 
Mr.  Lindgren  and  in  Europe  by  the  late  Professor  Stelzner,  and 
when  they  accept  neither  kaolinite  nor  sericite  on  faith  and  ex- 
ternal appearance. 

In  spite  of  the  mass  of  evidence  which  indicates  the  deposition 
of  a  large  class  of  gold  veins  by  highly  siliceous  alkaline  solu- 
tions, it  is  clear  that  some  deposits,  and  those  of  remarkable 
richness,  may  be  formed  by  acid  solutions.  To  explain  this 
essential  difference  in  the  vein  solutions  is  one  of  the  most  ab- 
sorbing problems  connected  with  ore  genesis.  It  is  possible  that 
emanations  from  a  crystallizing  magma  may  be  normally  and 
initially  acid  but  become  modified  by  passage  upward  through 
the  rocks.  If  there  is  any  truth  in  this  suggestion,  then  ores 
such  as  those  of  Goldfield  are  deposited  comparatively  near  to 
the  source  of  the  solutions.     Of  course,  the  extent  to  which 


692  FREDERICK  LESLIE  RANSOME 

initially  acid  emanations  would  be  neutralized  and  modified  in 
their  ascent  through  fissured  rock  would  depend  not  only  upon 
distance  but  very  largely  upon  the  kind  of  rock  traversed,  the 
quantity  and  character  of  admixed  surface-derived  waters,  and 
the  pressure  and  temperature  gradients. 

It  may  also  be  pointed  out  that  the  constituents  removed  from 
the  Goldfield  rocks  are  those  which  are  deposited  extensively  in 
the  production  of  gold-quartz  veins  of  the  sericitic  and  calcitic 
type.  Consequently,  conditions  are  easily  conceivable  under 
which  the  acid  solutions  of  Goldfield  might  have  ascended  through 
a  much  thicker  series  of  rocks  and  g^ven  rise  to  sericitic  and 
calcitic  veins. 

Finally,  the  relation  of  the  alunite  and  kaolinite  at  Goldfield 
suggests  the  attractive  possibility  of  the  future  discovery  of 
metasomatic  veins  of  the  pure  alunitic  type,  without  kaolinite. 

While  the  available  facts  in  the  Goldfield  district  appear  to 
justify  the  conclusions  drawn  in  this  paper,  the  cautious  reader 
may  well  share  with  the  writer  a  certain  reserve  in  judgment, 
in  view  of  the  moderate  depth,  about  300  feet,  to  which  study  of 
these  deposits  has  necessarily  been  limited.  The  possibility  that 
more  than  one  process  has  contributed  to  the  concentration  of 
the  sulphide  ores  and  to  the  alteration  of  the  rocks  is  one  that 
cannot  be  entirely  eliminated  until  the  mines  have  been  opened 
to  greater  depths. 


